Christina Mathias

Modelling carbon-water balance at tree scale in Eucalyptus plantations subject to water and nutrient stress

Thesis defended on 5 June 2015 at the Gaia Doctoral School

Global climate change is likely to increase the periods of drought in many tropical regions. Large quantities of potassium (K) are applied in tropical plantations but little in known about the consequences of interaction between K nutrition and water (W) stress on the physiological mechanisms governing tree growth.

The process-based MAESPA model of forest canopy radiation absorption, photosynthesis and water balance was used to study the combined effects of K deficit and W stress on the use of carbon (C) and water in monoclonal Eucalyptus plantations. The results showed that a reduction in K and W input reduces the gross photosynthesis and light use efficiency, with considerable interaction between K and W on C flows. The capacity of the tree root system to take up water appeared to be an essential factor in adapting to drought. There was significant water uptake below a depth of 10 m (including uptake from the water table), particularly during the dry seasons. Consequently, the Eucalyptus plantations management practices have a major effect on water resources and the level of the water table on the study site. However, simulations show that this effect may be limited by reducing the quantity of fertilisers applied. Inputs limiting the tree growth can increase the water resources in the soil throughout the rotation. The MAESPA model was modified (followed by a sensitivity analysis) to improve the simulation of leaf temperature, which is essential for simulations of climate change scenarios.

This thesis suggests that, in a context of climate change, fertiliser use should be reviewed in tropical rapid-growth plantations to make trees more drought-tolerant.






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